A water purifying device of the type described hereinabove is already known from German patent 38 14 683. It is also known from this that often the consumer and user of such a water purifying device does not replace the purifying medium in good time. For example, the ion exchangers often used to have a capacity to purify approximately 60 to 100 litres of water. In a normal household a water purifying device or preparation device of the type described in the introduction can be used perfectly well in a hygienic manner for a period of, for example, one month with one and the same purifying means. For reasons of hygiene, however, the purifying means, for example the ion exchanger or a mixture including it, should not be in use for very much longer than the lifetime indicated. The purifying medium should be replaced thereafter. It would thus be desirable to provide an indication at that time, showing a time reference, and to be able to replace the purifying medium in good time.
With previous filter cartridges, and also those of the type described in the patent described hereinabove, the problem of ventilation was significant, so a cap-like top part was provided by means of which the consumer could, at least in a mechanical way, be given a means for said consumer to recognise when a pre-determined time had elapsed, that is to say when it is expected that the purifying medium in the filter cartridge is exhausted. The top part, configured in a cap-shape, was provided with a perforated plate, wherein each hole was assigned to a month of the year. The consumer can place a pointer, for example a stopper attached to a band, in a specific hole at the beginning of use, in order to later remind himself of the time when, at the latest, the purifying medium in the filter cartridge ought to be exhausted. Disadvantageously, however, it was left to the consumer to place the stopper in one of the many holes. Disadvantageously, the stopper could also be knocked out of the previously correctly specified hole (for example, during use) and then the actual time for replacement of the filter cartridge was not known.
Tests have also already been carried out replacing these mechanical means with electronic indicators. A reed switch was built into an indicator, which switch interacts with a magnet arranged on the rim of the collecting jug or the like to the effect that each time the lid is removed, the magnetic field is interrupted and the reed switch actuated, so a kind of indirect volume measurement, optionally also linked to a time measurement, can provide information about the state of exhaustion of the purifying medium. Disadvantageously, the time remaining until the purifying medium is exhausted is reduced each time the lid is lifted off and put on, that is to say the indicator in this test also counts when movements of the reed switch relative to the magnets take place. Even when the lid is rotated, unintentional counting can result. With this test apparatus, the indicator with the reed switch must also be arranged in operating distance of the magnet. In one embodiment, fixing of the indicator in the edge of the lid was provided.